I find it difficult too, the variety, well I did when he couldn't have milk/soy. My son cannot also have wheat, oats, and barely, so all he could eat was rice. Honestly he loves waffles and bread that I get in the Organic Frozen section of the grocery store. The waffles are made with corn flour, he can have corn, and the Rice bread contains no dairy and no soy. They also have other breads made with wheat or oat, still no dairy and no soy and no eggs.
He also likes these Alphabet French Fries, from the organic section as they only have olive oil on them. He loves the rice crackers, rice cakes. I found I made a lot of homemade bread, with rice milk.
Your son is still young, so just even having fruit and vegs is really good for him. You can make finger foods with fruits and veg. I took sweet potato and made wedges, put some olive oil on them, baked them and he eats them. There are many recipes at
http://www.kidswithfoodallergies.org/recipes.html , you just tick off the things he cannot have. Hopefully that helps. I know it's a struggle. I still struggle because my son also had an easy gag reflex and wouldn't eat a lot of solid foods. He still eats a ton of veg and fruit, pureed meat, but is now trying the french fries homemade, waffles and other baked goods I either make myself or buy in the organic section.